CIRCLE OF WILLIS ​

INTRODUCTION : ​

● It is a hexagonal arterial circle, situated at the base of skull in the interpeduncular fossa. ● It is formed by the anterior cerebral branch of internal carotid, terminal part of internal carotid and the posterior cerebral branch of basilar . ● It is a circulatory that supplies to the brain and surrounding structures.

FORMATION : ​

ANTERIORLY : Anterior communicating artery joining the 2 cerebral arteries.

ANTEROLATERALLY : anterior cerebral arteries.

LATERALLY : Internal carotid arteries.

POSTEROLATERALLY : Posterior communicating arteries.

POSTERIORLY : Posterior cerebral artery.

BRANCHES :

The branches of the circulus arteriosus are cortical, central and choroidal. 1. Cortical or external branches run on the surface of the cerebrum. ​ 2. The central branches perforate the white matter to supply the thalamus, the corpus ​ striatum, and the internal capsule. 3. Choroidal branches supply the choroid plexus of the various ventricles. ​

CORTICAL BRANCHES :

These branches arise from all three cerebral arteries: ● Anterior cerebral ● Middle cerebral ● Posterior cerebral.

1. MIDDLE CEREBRAL ARTERY : It is the direct branch of internal carotid artery. ​ CORTICAL BRANCHES : ● orbital ● Frontal ● Parietal ● Temporal.

2. ANTERIOR CEREBRAL ARTERY : ​ It Is the smallest terminal branch of internal carotid artery. CORTICAL BRANCHES : ● Orbital ● Frontal ● Parietal.

3. POSTERIOR CEREBRAL : ​ It is the terminal branch of basilar artery. CORTICAL BRANCHES : ● Temporal ● Occipital ● Parieto occipital.

CEREBRAL CORTEX : Cerebral cortex is supplied by all the 3 arteries. ● SUPEROLATERAL SURFACE : This surface is mainly supplied by middle cerebral artery. ● MEDICAL AND TENTORIAL SURFACE : This surface is supplied by anterior cerebral artery. ● INFERIOR : Medial one third of orbital surface is supplied by anterior cerebral. Lateral two third, including the Temporal pole area and anterior surface of the Temporal pole is vascularised by middle cerebral artery.

CENTRAL BRANCHES : ANTEROMEDIAL arise as one group from both anterior cerebral and anterior communicating artery. ANTEROLATERAL arise in two groups one from each middle cerebral artery. These divide in two sets: medial striate and lateral striate. POSTEROMEDIAL arise in one group from posterior cerebral and posterior communicating artery. POSTEROLATERAL arises in two group from lateral part of the posterior cerebral artery.

CHOROIDAL BRANCHES :

● ANTERIOR CHOROIDAL is a branch of internal carotid artery. ● POSTERIOR CHOROIDAL arise from the posterior cerebral. ● POSTERIOR INFERIOR CEREBRAL ARTERY supplies the choroid plexus of 4th ventricle.

IMPORTANCE :

The circle of Willis allows equalization of blood flow between the left and right cerebral hemispheres, and can allow anastomotic circulation if parts are occluded. That is, the circle serves as a back-up system or a bypass, allowing for an alternative route if there is an occlusion in the normal route of supply to an area. For example, if there is an obstruction of blood supply through the left internal carotid artery, and blood cannot reach the front of the left side of the brain through this artery, blood will be routed to this area, through the anterior communicating artery, from the right internal carotid artery.

CLINICAL SIGNIFICANCE : Subclavian steal syndrome ​ The redundancies that the circle of Willis introduce can also lead to reduced cerebral perfusion. In subclavian steal syndrome, blood is "stolen" from the circle of Willis to preserve blood flow to the upper limb. Subclavian steal syndrome results from a proximal stenosis (narrowing) of the , an artery supplied by the , which is also the same that eventually feeds the circle of Willis via the vertebral artery.

Thrombosis of lateral striate branches of middle cerebral artery causes motor and sensory ​ loss to most of the opposite side of the body except lower limb.

Thrombosis of Heubner’s recurrent branch of the anterior cerebral artery causes ​ contralateral upper monoplegia.

REFERENCE : ​ BD Chaurasia's Human Anatomy